1. Field of the Invention
The present invention relates to a high strength steel sheet having a tensile strength not less than 40 kgf/mm.sup.2 and a high press formability which is suitable for the use as an interior and exterior sheets for automobiles and a method of producing the same.
2. Description of Related Art
High strength steel sheets have been hitherto used for body constructing members, outer panels and the like of automobiles in order to reduce weight of an automobile body. Such high strength steel sheets for automobiles are required to have necessary press formability and a sufficient strength for ensuring safety of automobiles at the same time. In addition, recently, under a situation that the regulation of total emission of exhaust gas is being considerably enforced, there is a pressing need to contemplate to provide a high strength steel sheet having a higher strength in future.
On the other hand, these steel sheets are sometimes subjected to heat treatment at not less than 900.degree. C. in order to eliminate distortion caused by forming or to increase secondary forming brittleness resistance, or heated to a high temperature due to welding, brazing or the like, so that it is also desired to have a property being hardly softened under such heating at a high temperature.
Further, from a viewpoint of a rust preventing property which has been recently considered to be especially important, it is desired to be a steel sheet with which various platings can be easily carried out.
Characteristics required for a high strength steel sheet having a high formability which is suitable for automobiles can be listed as follows.
(1) a ductility is high,
(2) an r-value is high,
(3) an yield ratio is low, and
(4) an in-plane anisotropy of a material quality is small.
With respect to them, for example, there are disclosed a cold rolled steel sheet adapted for press forming in a large dimension which is excellent in rigidity (high Young's modulus) and a production method thereof in Japanese Patent Application Laid Open No. 57-181361, and a method of producing a cold rolled steel sheet for deep drawing having a slow aging property and a small anisotropy in Japanese Patent Application Laid Open No. 58-25436, respectively. In both of them, an extra low carbon steel is used as a base material, Nb, Ti and the other are added in a trace amount, and further continuous annealing conditions are controlled. Furthermore, phosphorus is used as a strengthening element in order to provide high tensile force since it gives less deterioration of material quality and has a large ability for strengthening solid solution. However, the limit of a tensile strength of this P-added extra low carbon steel is about a grade of 40 kgf/mm.sup.2 at most, and it is clear that a component system using the extra low carbon steel added with the solid solution strengthening element will become difficult in adaptation thereof for requirements of high strength of steel sheets on account of improvement of automobile body weight to be light which is considered to proceed rapidly in future.
In addition, with respect to the in-plane anisotropy which is considered to be subjected to stronger requirements in future, there is a description in the above mentioned Japanese Patent Application Laid Open No. 58-25436, however, they have a low tensile strength such as 30 kgf/mm.sup.2.
Other than the P-added solid solution strengthened steel sheet using the extra low carbon steel base as described above, as high tensile force steel sheets having different strengthening mechanisms are a transformation structure strengthened steel sheet (dual phase strengthened steel sheet), and a precipitation strengthened steel sheet.
Among them, the transformation structure steel sheet is easy to obtain a low yield ratio and excellent elongation, but it is not suitable for deep drawing because of a low r-value.
On the other hand, the precipitation strengthened steel sheet, namely a so-called HSLA (High Strength Low Alloy) steel sheet, is a steel in which Si, Mn, Nb and the like are added wherein solid solution strengthening of Si and Mn and strengthening owing to precipitation of a carbon nitride of Nb and strengthening owing to grain refining caused thereby are utilized, which is used for home electric appliances as well as for automobiles, however, a fault of this steel sheet is a high yield value, so that using conditions are restricted.
This precipitation strengthened steel sheet will be described hereinafter with following prior literatures.
There are disclosed a method of producing a high strength cold rolled steel sheet of the precipitation strengthened type in Japanese Patent Publication No. 54-27822 and a method for producing a high strength cold rolled steel sheet for deep drawing in Japanese Patent Publication No. 55-16214. However, in any one of them, the yield ratio exceeds 70%, and a high value not less than 80% is presented in almost all cases.
Further, Japanese Patent Application Laid Open No. 55-152128 also discloses a method of producing a precipitation strengthened steel sheet, wherein a high strength cold rolled steel sheet having a low yield ratio and excellent formability is produced by means of continuous annealing, but not refer to deep drawability of the steel sheet at all.
Furthermore, as to low C level Ti-IF (Interstitial Free) steels, Japanese Patent Application Laid Open No. 57-35662 discloses a cold rolled steel sheet for ultra-deep drawing which is excellent in secondary formability and Japanese Patent Application Laid Open No. 60-92453 discloses a cold rolled steel sheet for brazing and welding which is excellent in deep drawability. However, the tensile strength of the cold rolled steel sheet is less than 40 kgf/mm.sup.2 in Japanese Patent Laid-Open No. 57-35662 according to an example thereof, which does not reach the target tensile strength level of 40 kgf/mm.sup.2 in the present invention. In addition, Si is an essential component in the present invention and a limitation range thereof is 0.1-1.2 wt %, whereas there is no definition of Si in claims of Japanese Patent Application Laid Open No. 60-92453, and an Si content is not more than 0.09 wt % also in examples, so that it is essentially different from the present invention in which an effect of Si is effectively utilized.